Fix an update leaving Maestro unable to replace maestro-server - #329

After updating on Windows, Maestro could land in a state where no local session would start — reported as "the update becomes unusable because the replacement of the maestro-server failed, the service is still running while trying to replace the file".

What was happening

deployment_version() is <app version>-protocol-<n>, so every update invalidates the cached server binary and ensure_cached_binary re-downloads it. download_server_binary finished with a bare rename(tmp, dest) onto %APPDATA%\com.maestro.app\bin\maestro-server-windows-x86_64.exe.

Windows will not let the image of a running process be overwritten. Any surviving maestro-server.exe made that rename fail with ERROR_ACCESS_DENIED, and the error was fatal with no retry — download_server_binary → ensure_cached_binary → ensure_local_server → open_local_transport. The stale binary stayed in place, so relaunching just repeated the failure, and the .download-tmp leaked on every attempt.

A server survived quitting because nothing stopped it. kill_on_drop(true) only fires when the Child is dropped inside the runtime, and handle.exit(0) dropped neither ACP map. release_active_project_lock does clear connection_servers — its comment even claims that covers "leaving the project or quitting" — but the quit path never called it that way. An installer-forced kill or a crash leaves the same orphan with no chance to exit at all.

The fix

Make the swap survive a locked destination. Renaming a running image is permitted on Windows even though overwriting and deleting are not, so install_binary_at moves the old binary aside to <name>.old-<nanos> and lets the new one take its name — the same trick install_local_link already used for ~/.local/bin a hundred lines earlier. Deleting the aside copy only succeeds once its process exits, so sweep_stale_binaries reclaims it on a later run. Failure paths restore the old binary rather than leaving nothing at the path, and clean up the partial download.

Stop orphaning servers on quit. stop_connection_servers clears acp.sessions and acp.connection_servers in the CloseRequested handler, so kill_on_drop fires.

A graceful stdin-EOF shutdown was considered and does not work here: the shared reader task holds its own writer_tx clone until it sees EOF, so the pipe never closes and the wait would always time out. Clearing the maps is what the leave-a-project path has always done.

Testing

cargo test -p maestro --lib — 402 passed. cargo clippy --workspace --all-targets -- -D warnings clean.

Three new tests in deploy.rs. The Windows one reproduces the real failure rather than approximating it: it copies ping.exe into place as the destination and spawns it so a live process holds that image, asserts the plain rename fails so the test proves something, then asserts install_binary_at succeeds anyway, leaves exactly one .old- sibling, and that the sweep reclaims it once the holder is killed.

Not yet verified by a manual GUI run that no maestro-server.exe survives a normal quit; that half is verified by inspection against the release_active_project_lock path it mirrors.

Release Notes:

• Fixed an update on Windows leaving Maestro unable to start sessions when a leftover maestro-server process still held the old binary

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